There’s a fair few decent built descents in the hills up behind Darvel/Newmilns, best bet is to message sprockets cycles and go on a shop ride, Dave builds up there and knows it all, You can either climb up the concrete road from Darvel, or there is a spot you can park off the back road from Galston to Sorn that takes a few mins to get to the top of the woods, I’ve not rode there for a while, but I’ve put a strava ride below that’ll give you a heads up.
https://www.strava.com/activities/3014542062#74776686771
Decent wee loop around Mauchline, River Ayr and Catrine, there’s a load of wee short fun trails in the woods down by Ballochmyle golf course, you’ll pass them on this loop.
https://www.strava.com/activities/419012389#10079913304
Loch Doon trails are pretty good, but will be baw deep in muck at present!
https://www.strava.com/activities/1623805121#40665258313
Auchincruive is my local fun spot, some decent other tracks up the river, be happy to show you around if you fancy it. Ride below was a day building, chequered flag is where all trails start.
https://www.strava.com/activities/4053683866
VMGScot is spot on, four lochs is cracking, big wild country, but actually fairly compact, you just feel as if it’s a massive area. You could also ride this area and Loch doon area from Afton Reservoir if you’re up for a big gravel ride, wind farm roads take you right over to Carsphairn, then 5 ish miles on the road up to Eriff, then you can cut over to the Head of the Loch. Apart from one wee 10 minute carry (I’d advise using a mapping app for this wee bit) you can actually ride right around Loch Doon now, I did it as a training ride for WHW in a day in the summer.
https://www.strava.com/activities/3745762066
Of course, Fairlie Moor is about my fave local spot, but none of us can ride that at the moment! 🙁